1. Field of the Invention
This invention relates to apparatus for sampling air, and, more particularly, to apparatus for sampling air over a wide dynamic range.
2. Description of the Prior Art
U.S. Pat. No. 4,481,829, dated Nov. 13, 1984, discloses basic prior art apparatus, which includes a base frame and a hood securable to the base frame for measuring air flow for the purpose of balancing air flow systems. The '829 patent also discloses different types of pressure sensing grids disposed within the frame. Baffle systems are also disclosed for use with the pressure sensing grids to enable relatively low air flows to be measured.
The apparatus of the present invention may be considered as a second generation to the apparatus disclosed in the '829 patent.
The advent of integrated circuits has encouraged the use of microprocessors for numerous control and calculating functions. Microprocessors are used in the apparatus of the present invention, along with other electronic elements and with mechanical elements.
Prior art air measuring apparatus is generally limited in the data provided. For example, so far as is known, none of the prior art devices is able to provide back pressure compensation, and none is able to correct for air density. Moreover, none is able to measure relatively low air flows, and none is able to measure air flows over a relatively wide dynamic range. The apparatus of the present invention measures air flows from as low as about twenty-five or thirty feet per minute air velocity up to about twenty-five thousand feet per minute air velocity. The latter dynamic range represents a turn-down range of about one thousand to one, or a range in terms of inches of water column from about 0.00004 inches to about fifty-eight inches.
The apparatus of the present invention overcomes the limitations of the prior art by automatically compensating for back pressure and for automatically correcting for air density. Accordingly, air measurement data are correct wherever the apparatus is used, whether at sea level, below sea level, or above sea level.
Relatively low air flows may be measured with a hot wire anemometer, but the same hot wire element may not also be used to measure relatively high air flows. Also, since a hot wire anemometer requires heat, the hot wire element interferes with its own environment by creating its own convection currents. These convection currents interfere with the flows being measured, particularly when the flows are relatively small. That is, the smaller the air flow being measured, the greater the interference with the air flow caused by the hot wire elements.